[Regulatory T cells and regulatory NK cells play essential roles for maintenance of pregnancy].

During pregnancy, semiallograftic fetus is allowed to grow without being rejected by the maternal immune system. Recent data show that paternal antigen specific- or male antigen HY specific-regulatory T cells (Treg) increase during pregnancy. Seminal plasma is necessary for the accumulation of Treg cells in uterine draining lymph modes just before implantation, and in uterus just after implantation. It has been reported that decreased number of Treg cells in peripheral blood or pregnant uterus in abortion or preeclampsia. In human and rodent pregnant uterus, NK cells are the major population (∼80%) of lymyhocytes, we have reported that CD25(+) NK cells increase in mouse pregnant uterus, and these NK cells produce immunoregulatory cytokines such as IL-10 and TGF-β. They inhibit the expression of MHC class II antigen on dendritic cells and inhibit the cytotoxic T cell induction, suggesting that these NK cells have an ability for immunoregulation. Both Treg cells and regulatory NK cells play important roles for maintenance of pregnancy.